This disclosure relates to a process for measuring optical properties in a transparent material and more particularly, to a process for quantitatively measuring ripple and distortion present in the transparent material.
During the manufacture of transparent materials, optical defects and deviations may be produced that render the transparent material optically imperfect. Optical imperfections are of special concern in glass and plastic sheet applications where optical defects are unacceptable from a quality control standpoint. Optical quality defects include inconsistencies in light transmission and/or bending of light that distorts the physical dimensions of the viewed object. Ripples, distortion, gels, scratches, particles, die lines, and grind lines are common categories of optical quality defects.
Ripples are commonly found in transparent materials formed from continuous web processes. Ripples are generally characterized as a periodic distortion wave that orients itself across the web. Ripples may be caused by oscillations in calendaring roll speeds (i.e., gear ripples), release dynamics from the calendaring rolls and other transient oscillations in force (i.e., stick ripples), temperature fluctuations that occur between 0.1 and 100 hertz depending on the line speed, roller bearing inaccuracies, resonance oscillations of the roll or roll stack, motor speed control, bead dynamics, chill marks, pull roll inconsistencies, oscillations from shear, and the like. Ripples may be very sharp and distinct or may be fuzzy at the edges (from distortion) but are generally defined by their consistent cross web orientation.
Distortion also refers to waves. However, the waves presented show less directional orientation than ripples and as such, can be either in the cross web direction or in the machine direction. Distortion in extruded sheet materials most often is caused by variability in the cooling rates resulting in localized variations in the index of refraction for the transparent sheet material. Heat distortion patterns are often not easily separated from ripple patterns since heat distortion often orients with ripples. For this reason, these ripple and distortion are typically evaluated together.
Distortion is mathematically defined as the rate of change of the angular deviation of a light beam across a transparent sheet (or film). Distortion in a transparent sheet may be determined by mapping, at a plurality of locations on the transparent sheet, the angular deviation of light beams as they are transmitted from the light source through the transparency to the observer.
Angular deviation is different than the normal lateral displacement (lateral shifting) of the light beam as it passes through the transparent sheet material. Whenever a beam of light passes through the transparent sheet material at an angle other than perpendicular to the face of the sheet, lateral displacement of the beam by a relatively small and constant amount results. However, when a beam of light passes through the transparent sheet at the same angle, but where the faces of the transparency are non-parallel, both lateral displacement and angular deviation result. While lateral displacement is generally insignificant in automotive windshield applications and the like, the angular deviation and distortion causes the distance between the real location of the object and its apparent (angular deviation) position to increase as the range of the object from the observer increases. Consequently, a method of quantifying the angular deviation and the distortion in a transparent material to ascertain the severity of the distortion is beneficial in quantifying the acceptable limits of ripple and distortion for the intended application.
Distortion is typically analyzed based on the non-linear mapping of objects viewed through the transparent sheet, such that the actual physical position of an object does not linearly correspond to its apparent location as seen through the glass or other transparent sheet. In the case of a perfect non-distorting transparent sheet, a square grid object would be reproduced as an identical square grid in the image plane. Where there are wedge variations, curvature variations, optical index defects, or the like, in the transparent sheet, the square grid will be reproduced in a distorted form in the image plane.
The existing methods and systems for detecting and measuring distortion are difficult to use, time consuming, and unreliable. Many of the test systems involve some form of manual inspection procedure. A conventional inspection procedure involves a visual inspection from a distant position of a stripe pattern or grid pattern through the transparent sheet. An inspector in viewing the pattern can visually determine the optical quality. The zone with the largest defect is located and a measurement of the distortion is made where the distortion is the greatest. Reference transparent sheets may be used to arbitrarily assign a value to the test transparency. However, real quantitative standardization cannot be obtained and differences among inspectors prevent uniform inspection quality.